1. Field of the Invention
This invention relates to a GaAs system high-output epitaxial wafer for infrared light-emitting diodes.
2. Description of the Prior Art
Wafers on which a GaAs or GaAlAs p-n junction is epitaxially formed on a GaAs substrate are extensively used for infrared light-emitting diodes (LEDs) used as light sources in photocouplers, remote controllers, and other such devices. Examples of the use of GaAs p-n junctions are described in National Technical Report, Vol. 18, No. 3, 1972, pages 249 to 258, and in JP-A 59-121830, for example. Examples of the use of GaAlAs p-n junctions include those described by the Journal of Applied Physics, Vol. 48 No. 6, Jun. 1977, and JP-A 6-21507. In an LED comprised of GaAlAs formed by adding Al to the GaAs p-n junction, the emission wavelength has been adjusted towards the short wavelength side of the spectrum. Among such infrared light-emitting diodes, there are those, such as the ones described by JP-A 59-121830 and JP-A 6-21507, with a window structure that provide a high output and are increasingly in demand for remote control applications. These window type light-emitting diodes are comprised of p-n junction on a GaAs substrate, which is then overlaid with a window layer of Ga.sub.1-z Al.sub.z As having a larger bandgap, confining injected electrons through the p-n junction into a p-type active layer where the light is generated. The transparency of the Ga.sub.1-z Al.sub.z As window layer to the emitted light is utilized to increase the external light output efficiency. (See for example "Fundamentals of Semiconductor Devices" by E. S. Yang, 1978, pp168-169, McGraw-Hill.)
In such light-emitting diodes, single-step liquid-phase epitaxy using the spontaneous dopant Si is used to form the GaAs or GaAlAs p-n junction region in a continuous process. This is because when GaAs is formed by liquid-phase epitaxy with Si as a dopant, high temperature formation results in n-type crystal, and low temperature formation results in p-type crystal. This means that with the same epitaxy solution, it is possible to grow an n-type layer and a p-type layer in a continuous operation, so productivity is high and the resultant p-n junction has good crystallinity. The emission wavelength of GaAs doped with Si depends on the concentration of the Si, but typically is from 930 to 950 nm, a wavelength that is particularly suitable for remote control light source applications.
In the GaAs the Si forms a deep acceptor level. Light emission is produced in the p-GaAs active layer by the recombination process in the region between the GaAs conduction band and the Si acceptor level. As a result, with a peak emission wavelength of from 930 to 950 nm, the emission energy of Si-doped p-GaAs is less than that of the GaAs bandgap (1.42 eV, corresponding to a wavelength of around 870 nm).
For a window layer, a material is used that has a higher bandgap energy than the active layer. GaAlAs mixed crystal, formed by adding Al to GaAs, has a higher bandgap energy than GaAs, and when used as a window layer for a GaAs active layer, effectively confines injected electrons and is transparent to the emitted light. Also, a window effect can be provided using GaAlAs having a higher Al content compared to the GaAlAs active layer. Accordingly, a high Al content GaAlAs window layer is used in GaAs and GaAlAs LEDs. When a window layer has the same conduction type as the active layer and the active layer is formed by the single-step growth process using Si, a window layer in which Si is used as the dopant is widely employed. This is because Si has a low vapor pressure at a temperature around the epitaxial growth temperature, which makes it relatively easy to handle, and carrier concentration in the window layer can be controlled with good precision.
A Ga.sub.1-z Al.sub.z As window layer has to have a composition that does not absorb emitted light from the active layer. In the case of a Ga.sub.1-z Al.sub.z As window layer in which Si is used as the dopant, emitted light is absorbed toward the short wavelength side from the wavelength corresponding to the energy between the Ga.sub.1-z Al.sub.z As conduction band and the Si acceptor level. If z is 0.1, the wavelength corresponding to the energy between the Ga.sub.1-z Al.sub.z As conduction band and the Si acceptor level is around 860 nm. In the case of a GaAs active layer (that is, Ga.sub.1-y Al.sub.y As, when y=0), with a z of 0.1 or more substantially none of the light will be absorbed by the window layer, but with a z that is less than 0.1, light starts to be absorbed by the window layer. As such, when a GaAs active layer is used it is necessary for z to be at least 0.1 over the entire window layer to prevent absorption by the window layer. In practice the window layer is formed by the liquid-phase epitaxial method, and because of the high segregation coefficient of Al, z is largest at the interface with the active layer, gradually decreases toward the epitaxial layer surface, and is at its smallest at the window layer surface. Therefore the z value is set at not less than 0.1 at the interface with the p-type epitaxial layer. Based on the same type of consideration, when the active layer is formed of Ga.sub.a-y Al.sub.y As (where y.noteq.0), the Al mixing ratio z at the surface of the window layer has to be at least 0.1 more than y to prevent absorption by the window layer.
However, making the Al mixing ratio z at the surface of the window layer larger than 0.1 produces a rise in the forward voltage, one of an LED's important characteristics. In an LED a low forward voltage is preferable. For LEDs that are used as the light source for remote control applications, a low forward voltage is particularly important owing to the fact that they are battery driven. When Si is used as the window layer dopant, it is extremely difficult to reduce the forward voltage while at the same time preventing any absorption by the window layer.
When Si, which is an amphoteric dopant, as mentioned above, is used as the dopant in a p-Ga.sub.1-z Al.sub.a As window layer, epitaxial growth conditions are used so that p-type Si is produced. However, in the initial stage of the window layer formation process, small, local variations in the growth conditions can readily cause an n-type conductivity inversion in parts. Parts that have undergone n-type conductivity inversion are defective and reduce product yield.
When a GaAs p-n junction is formed using the single-step growth process using Si, carder concentration in the vicinity undergoes a major decrease. Thus, even a slight change in the growing atmosphere can result in the formation of a zigzag junction (105), as indicated in FIG. 4 by arrow (b), degrading device characteristics. A zigzag junction extends across the entire wafer and is several tens of .mu.m in size, although large ones may measure several hundred .mu.m. An LED epitaxial wafer is cleaved into chips after electrodes have been formed on each surface of the wafer. The chips are generally 250 to 350 .mu.m wide. A normal p-n junction is like the clear p-n junction indicated by arrow (a) in FIG. 4. When a chip is within a large zigzag junction, the result is a p-n-p-n junction extending from the p-GaAs layer (103) to the n-GaAs layer (102), indicated in FIG. 4 by arrow Co), and a device that does not function properly.
An object of the present invention is to provide an epitaxial wafer for a light-emitting diode with a window layer in which Si concentration is kept low and in which there is substantially no light absorption by Si dopant in the window layer.
Another object of the present invention is to provide an epitaxial wafer in which slight variation in epitaxial process conditions used in p-n junction formation does not give rise to inverted p-n portions, reducing the formation of a zigzag junction in the vicinity of the p-n junction.
Still another object of the present invention is to provide an epitaxial wafer in which a p-GaAlAs window layer is prevented from being partially inverted into an n-GaAlAs crystal.
A further object of the present invention is to provide an epitaxial wafer having a low forward voltage and a high emission efficiency without increasing the window layer surface Al mixing ratio.